Question regarding ballistics programs

knockturnal

knockturnal

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So, I just an update for my ballistics program, and it now has g1-g8 ballistics. Which is great cause now my loads actually seem quite normal. Anywho, my question is regarding the sight height. How in Sam hell are you supposed to measure the sight height properly? Defalt is 1.5" but is that supposed to be from the top of the barrel? Or from the middle of the barrel? I'm pretty sure mine is higher then 1.5.
 
scope height is centerline of bore to centerline of scope.

Just put the basic load/bullet data in, get outside and shoot. You will have to tweak it no matter.

Makes sure the program will allow you to adjust some data points then modifies the chart to fit the new points. If not, print and just dial it in manually.

Eventually, you will have a drop chart specific to your rifle, bullet, load, scope, and rangefinder.

Depending on the bullet, G7 is the better bullet drag function to use. See the Litz book.
Jerry
 
mysticplayer said:
See the Litz book.
Jerry
Click to expand...

+1. Great book that answers a lot of questions about the nuts and bolts of BC, references shapes, etc. and shows how to get the most from your ballistics program.

As for the 20MOA base, that is only 0.33 degrees (unless my math is faulty, 60MOA = 1 degree, right?), which works out to around a 0.070" difference over a 14" long scope. It won't matter to the program if you are off by that much, just measure at the objective lense and you will be fine. The only big influence that scope height has is on maximum point blank range when setting up a rifle for hunting. For fixed distance target shooting it makes no difference at all.

Mark
 
Bore centreline to scope centreline. No need for any degree of precision here, eyeballing it to the nearest half or quarter inch really is good enough. Do some runs with height=1.5" and height=2.0" and see for yourself.

Sloped scope base won't matter.

Different parameters have different sensitivities to error. Roughly, in order of most sensitive to least, are:

- muzzle velocity. Chrono'ing very worthwhile. Only guess from load data if you really truly can't measure. Optionally, you can adjust chrono data (at 10' or 15') to true muzzle velocity, though this is a fairly small effect.

- bullet BC. Manufacturer's data a good start, but be aware that manufacturer X versus manufacturer Y are definitely not working on apples-to-apples basis. Better yet is Litz's data, either G1 or G7, if available for your bullet - see JBM web ballisitics program (free!) or Litz's book (not free, but fantastic!).

- elevation above sea level (affects air density) - use the elevation of the nearest airport, or have a look at Google Maps contour lines. Nearest 100m is good enough

- temperature (affects air density). Nearest 5C is good enough for most purposes.

- humidity (very small affect on air density). Choose something from 50% to 100% that is representative of your local climate. Quite insensitive for most purposes.

- bullet drag model (G1 vs. G7) - if using a high performance boattail target bullet, at long range (800+ yards), using a G7 b.c. will give you a slightly truer figure.
 
Take the stock off your rifle.

Measure the outside diameter of the barrel (near the objective lens) and divide by 2.

Measure outside diameter of objective tube and divide by 2.

Measure the distance between objective tube and barrel.

Add all three number up and you should have a pretty close number.
 
mysticplayer said:
Depending on the bullet, G7 is the better bullet drag function to use. See the Litz book.
Jerry
Click to expand...

As you know the key to making this all work properly is having a program that allows the user to enter multiple BC data for changes in velocity. Whether multiple G7 data provides better drop information vs. the G1 data is up for debate. I continue to see good results with the multiple G1 data all the way out to 1 mile. If I'm out on elevation it's not by much and probably has more to do with incorrect environmental readings rather than the BC data I've used. But then again...ymmv.
 
ryandpahl said:
Take the stock off your rifle.

Measure the outside diameter of the barrel (near the objective lens) and divide by 2.

Measure outside diameter of objective tube and divide by 2.

Measure the distance between objective tube and barrel.

Add all three number up and you should have a pretty close number.
Click to expand...

I don't know why I didn't think of that. Haha. Brain fart.

And according to the Litz book, the g1-g7 ballistics remain somewhat the same until you really start stretching it out there. I've compared the g1 to the g7 with my loads and get a difference of roughly 90" at2000 yards. Which is quite a bit.
 
well, i just spent the last 2 hours playing around with JBM, the Litz program and iSnipe. Ive only been able to come close to what my rifle shoots with iSnipe. Ive got it saved and am going to try it out this weekend after i load some rounds. See how accurate it is. Thanks for the tips on measuring scope height.
 
Are you shooting (or interested in shooting) at extreme ranges of well past 1000 yards? If so, then you do need to pay attention to every little detail, if you want to predict thing as closely as possible

If you are content with predicting things after the fact, which is actually not quite as useless as it sounds ;-), you can effectively end up "back-calibrating" your situation and be able to get pretty useable results in the future. For example, given an initial "calibration session" at your range with a particular load, by fiddling with your bc. you can make your ballistics program match your observed results. Then on another day when you have different atmospheric conditions, or a different muzzle velocity, you can predict your new sight settings pretty closely.

Another thing not to be trusted (even for 1000yard shooting) and needing actual on-the-range measurement, is your actual scope adjustments. You can't assume that your scope moves 1 minute just because that's what it says on the dial. If it actually moves 1.02 minutes, or 0.97 minutes, that's likely one of your bigger error sources. When you do this measurement, you will not only need to measure your target to better than 1%, you will also need to measure the distance from your target to your rifle to better than 1% (for example, on my 600m range the "100m" firing point is actually 95m from the target).
 
knockturnal said:
well, i just spent the last 2 hours playing around with JBM, the Litz program and iSnipe. Ive only been able to come close to what my rifle shoots with iSnipe. Ive got it saved and am going to try it out this weekend after i load some rounds. See how accurate it is. Thanks for the tips on measuring scope height.
Click to expand...

i am not sure why you are getting so much difference in the programs? i run the nightforce exbal, litz and sierra. when we were shooting at 1 mile a couple weeks ago the sierra and litz were almost identical and the exbal was only different by 1 minute if i recall correctly.
 
Well, after yesterdays 2hr clusterfack of back and forth from one program to another, I think I'm just gonna say screw the program and shoot out to a mile, all while working up a range card. I've shot out to 1480 but totally lost track of how many Moa I adjusted. Which is my bad cause I forgot to black out my other hash marks on my turret. When I shot 2 weekends ago, I dialed up 14.5 moa to hit 900 yards on a 8degree angle. The same day, I dialed up 8.25 to hit 640. I always have my phone with me so I keep track of the ranges and Moa adjusted. I'm pretty much prepared from 100 to 900 yards with no problems. The only program that has come anywhere close to what I dial is iSnipe. Which is weird. Now I just need the snow to melt so I can get to the scud launch range in summerland to stretch it's legs once again.
 
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